Permanent rare-earth magnets have magnetic properties that are high enough to use them in various fields today. Those permanent rare-earth magnets are roughly classifiable into sintered magnets and bonded magnets according to the material powder to use and the manufacturing method. A bonded rare-earth magnet (which will be simply referred to herein as a “bonded magnet”) can be formed in any shape more flexibly than a sintered rare-earth magnet. In addition, as there is an insulator resin between the magnet powder particles of a bonded magnet, the bonded magnet has high electrical resistance, which is beneficial. On top of that, since a nanocomposite magnet powder which requires approximately two-thirds as large an amount of rare-earth metal as a sintered rare-earth magnet does may be used, such a bonded magnet is expected to attract more and more attention from now on as a permanent magnet that can save the amount of the rare-earth metal to use.
However, a bonded magnet includes a resin binder, and therefore, its magnet density is lower than an alloy true density (of 7.6 g/cm3, for example). And even a compressed bonded magnet, which generally has a small amount of resin binder, has a magnet density of 5.5 to 6.2 g/cm3 (which accounts for approximately 72 to 82% of the alloy true density). As a result, the magnetic properties of a bonded magnet cannot but be lower than those of a sintered rare-earth magnet.
A binderless magnet which is obtained by binding a magnet powder of a rapidly solidified rare-earth alloy (which will be simply referred to herein as a “magnet powder”) through a cold compression process under an ultrahigh pressure of 500 to 2500 MPa without using any resin binder was developed recently and disclosed in Patent Document No. 1. The binderless magnet disclosed in Patent Document No. 1 is said to have a density of 5.5 to 7.0 g/cm3 (which accounts for approximately 72 to 92% of the alloy true density). The binderless magnet includes no resin binder, and therefore, includes a magnet powder at a higher volume percentage than a bonded magnet does. Consequently, the binderless magnet can be expected to achieve magnetic properties that have never been realized by any bonded magnet.
To make a binderless magnet, however, the magnet powder needs to be subjected to the cold compression process and then thermally treated at a temperature of 350° C. to 800° C. so as to get sintered. Its magnetic properties actually deteriorate as a result of this heat treatment, and it is difficult to realize the magnetic properties just as intended. On top of that, in the binderless magnet, its magnet powder particles are bound together by producing solid-phase diffusion of atoms through the heat treatment at such a low temperature, and therefore, high mechanical strength cannot be obtained and its use will be a limited one. Furthermore, the binderless magnet uses no resin, and therefore, cannot achieve as high electrical resistance as the one achieved by a bonded magnet.
Patent Document No. 2 discloses a compressed bonded magnet which uses, as a resin binder, a resin that is in liquid phase at an ordinary temperature. Patent Document No. 2 says that a high-density bonded magnet can be obtained by using a magnet powder with good flowability and a particle body made of such a liquid resin binder. In an example of Patent Document No. 2, its density falls within the range of 6.2 to 6.4 g/cm3. However, since such a liquid resin is so difficult to handle that the productivity tends to be very low, it is hard to apply the method of Patent Document No. 2 to mass production.
Patent Document No. 3 says that if a solid resin is dissolved in an organic solvent an used when a compound is made during the manufacturing process of a bonded magnet, the surface of the magnet powder can be uniformly coated with the resin. Patent Document No. 3 also says that by compressing such a compound, a bonded magnet with excellent magnetic properties, mechanical strength, thermal resistance and corrosion resistance can be obtained. Patent Document No. 3 discloses a bonded magnet with a relatively high magnet density. In an example of Patent Document No. 3, the maximum magnet density is 6.3 g/cm3. According to Patent Document No. 3, however, to improve the magnetic properties, it is effective to increase the volume percentage of the magnet powder and, decrease the percentage of the resin accordingly. Nevertheless, Patent Document No. 3 also says that the percentage of the resin cannot be reduced to less than a certain limit considering the compactability and mechanical strength of the bonded magnet. And since the bonded magnet of Patent Document No. 3 includes 1.5 to 2.0 mass % of resin with respect to the magnet powder, it is difficult to achieve an even higher magnet density.